When using a massive Multiple Input Multiple Output (MIMO) system, a communication service provider can easily satisfy a high data rate, which is required in a next generation communication system after 4G, with only a simple linear precoder by providing a plurality of antennas in a base station. In theory, when using an infinite number of antennas, a communication service provider can perfectly remove various problems, such as fast fading, inter-user interference, and the like, which limit the performance of a system, by using a linear precoder.
When a multiple user system based on the massive MIMO is configured, throughput of the system, which is much higher than that of an existing communication system, can be obtained at a low cost. Such an advantage of the massive MIMO system is based on the assumption that the base station has identified channel information. In a situation where there are a large number of antennas, such an assumption is very burdensome to the system. Therefore, conventional researches on a massive MIMO system have been made based on a Time-Division Duplex (TDD) system in which a channel estimation cost is not affected by the number of antennas of the base station due to channel reciprocity.
Meanwhile, when data transmission amounts of uplink (UL) and downlink (DL) are similar to each other or when a distance between a transmission end and a reception end is long, the TDD system has a frequency efficiency lower than that of a Frequency-Division Duplex (FDD) system due to conversion between a transmission mode and a reception mode. For this reason, conventional communication systems, such as a Universal Mobile Telecommunications System (UMTS), a Wideband Code Division Multiple Access (WCDMA), a CDMA2000, and the like, have supported an FDD mode in various environments. Therefore, researches and development on an FDD massive MIMO system for securing backward compatibility could be considered to be a necessary and sufficient condition which allows the massive MIMO system to be a core technology of the next-generation communication system.
In the FDD system, channel reciprocity is not established. Thus, the FDD system uses a limited feedback system, which quantizes a channel estimated by the reception end and transmits the quantized channel to the transmission end, in order to secure Channel State Information at the Transmitter (CSIT). Meanwhile, conventional researches on the limited feedback show that the size of a codebook for restraining system throughput deterioration caused by the limited feedback in a single stream environment exponentially increases according to the number of transmission antennas. The size of a codebook needed when the environment of a system is expanded to a multi-stream (or multi-user) environment must increase according to a signal-to-noise ratio (SNR) as well as the number of transmission antennas. These facts suggest that the number of pilot signals, feedback channel capacity, the size of a codebook, and the like are main factors which restrain the throughput of the FDD massive MIMO system.
FIG. 1 is a diagram illustrating each modulation scheme.
According to the conventional modulation schemes, such as Quadrature Amplitude Modulation (QAM), Frequency-Shift Keying (FSK), and Phase-Shift Keying (PSK), entities associated with communication transmit and receive information with the sizes of symbols, frequencies, phases, and the like which are differentiated in a time domain or a spatial domain of a signal, as shown in FIG. 1. When a channel gain is high, when various types of interference are small, or when a large transmission power is used, an effective inter-symbol distance “d” at a reception end is lengthened, so that a transmission rate can increase by using a high-order modulation scheme. In a massive MIMO system, a communication service provider can efficiently overcome fading by hardening a channel gain to a path loss using a linear precoding scheme, such as Maximum Ratio Transmission (MRT), Zero-Forcing (ZF), Minimum Mean Square Error (MMSE) filtering, or the like. In addition, the communication service provider can efficiently remove inter-cell or intra-cell interference in the same manner. Therefore, when using a massive MIMO system, the communication service provider can provide a superior performance with the conventional modulation schemes, as compared with the case of using the conventional normal MIMO system.
The characteristic of the massive MIMO system which is robust against fading and interference can be acquired only when a transmission end is completely aware of channel information. In an FDD system, in which channel correlation can be used, a process of acquiring, by a transmission end, channel information may be a large burden on communication entities.
FIG. 2 is a diagram illustrating channel reciprocity in a TDD system.
In a TDD system, a base station can estimate the entire DL channels using UL pilots proportional to the number of antennas of a terminal on the basis of channel correlation, as shown in FIG. 2. In contrast, in an FDD system, since it is impossible to estimate a DL channel through a UL channel, pilot signals proportional to the number of antennas of a base station are required, and a channel estimated by a terminal must be fed back to a base station.
In the FDD system, generally, a transmission end uses a codebook-based limited feedback system, such as DFT or Grassmannian, in order to acquire channel information (CSIT). When a transmission end has one antenna (i.e. MT=1), the number “B” of feedback bits for having a performance difference of 3 dB from a system capable of using the full CSIT regardless of a signal-to-noise ratio (SNR) is expressed as equation 1 below.B=MT−1  [Equation 1]
Here, MT represents the number of transmission antennas, and the size of a required codebook is “N=2B”. This means that, when MT=16, a codebook having the size of “N=32768” or greater is required in order to reduce a performance loss to 3 dB or less with respect to the full CSIT. Meanwhile, when there are a plurality of transmission antennas (i.e. MT>1), a difference between the full CSIT according to transmission power ρ and the capacity of limited feedback system is expressed as equation 2 below.CCSIT(ρ)−CFB(ρ)<MT log2(1+ρ·2−B/(MT−1))  [Equation 2]
Referring to equation 2, it can be understood that, when MT>1, the number of feedback bits for constantly maintaining a difference in performance from the case of using the full CSIT increases by ρ as well as by MT. Since the performance of an FDD massive MIMO system is limited by the size of a codebook, as described above, the advantage of the massive MIMO system, such as channel hardening, cannot be provided when “N” is finite although MT is sufficiently large. That is to say, in the FDD massive MIMO system, it is impossible to easily ensure an effective inter-symbol distance “d” at a reception end through precoding, and thus it is impossible to increase a transmission rate in the conventional modulation schemes.